I have done it, or at least started the process. The Utah National Parks Council has a training schedule with discounted prices for scout leaders who want to do exactly what you want to do. I have not seen the schedule for 2014, but last year there were classes in the spring and fall.
There are different certifications as you know: Rifle, pistol, shotgun are the ones you would use most often in scouting. There are also Muzzle loading and reloading certifications you can do. Then there is also a Range Safety officer certification as well.
Every certification requires the NRA basic instructor guide. Last year the cost was $20 (I think that is right) for scout leaders, regular price $75. Then you have to pick the type of gun. Rifle, pistol and shotgun was around $80 and Muzzy was $85. If you are not a registered BSA leader it would cost $250 for each weapon. You only have to take the basic once, so if you want to do each weapon don't have to repeat the basic course every time, just the shooting section. You also have to pay NRA fees to be registered, which I want to say was around $20 bucks for the cheapest option.
I took the basic instructor class as part of the shotgun certification. The class was scheduled for 3 days. The basic was taught on Thursday night, then the shooting section Friday afternoon/evening and then finish on Saturday. I would imagine the rifle and pistol would be similar.
Now, you say that you want to be able to safely shoot when you are on campouts? This is something that almost never done the right way. Technically, the 100% correct way to do it is to have a Range Safety Officer present, as well as a certified instructor for each discipline. The RSO and the weapon instructor must be 2 different people. Most BSA leaders don't know this and it is widely ignored. In today's world I could see this as a huge liability issue that would not end well for the leaders involved if something were to happen during a campout and the rules were not followed correctly.
One more thing I want to share that I learned at the Basic course. I knew that the scouts were limited to shooting .22's, but I was not aware that they have to be bolt-action or single-shot rifles. Ruger 10/22 are not an approved gun to use, as they are a semi-auto.
Here is a link to the scout website FAQ. It actually has some really good questions and explanations on it: